Pumps are often used in conjunction with gases or liquids such as acids, oils, and toxins which can cause serious harm to the environment if they escape. Thus, when pumping a dangerous liquid from one location to another, it is important that neither the liquid nor gas which is often released by the liquid, escape to the atmosphere or pump areas outside the desired fluid pumping path.
In the 1930's mechanical seals were developed to overcome prior pump shaft sealing problems. This permitted a more secure seal against liquids from escaping along the shaft of the pump. However, in some cases liquid escaped when the pressure within the pump became too high for the seal to handle. The competing interests of maintaining an efficient pump and a safe pump required appropriately balancing the two requirements. Predicting the amount of safety required could only be broadly approximated based on the type of liquid to be pumped. The more hazardous the liquid, the more secure the seals.
The other problem with the mechanical seals of the 1930's was that the gases which were produced by the liquids were not always stopped. The seals were often easily permeated by the vapor. One solution to this problem was the creation of an arrangement known as a double seal with a barrier fluid protection. In this arrangement, the two seals form a cavity which is then filled with a clean fluid. The seal facing the excess liquid, that which does not exit the pump where desired, inhibits the movement of the liquid sufficiently to prevent passing of the liquid. The vapor which can permeate the seal is stopped by the clean fluid in the cavity.
One of the problems with this double seal system was that any failure by the first seal could defeat the protection system. Either gases from the liquid could then escape through the barrier to the environment or the liquid could break through the second seal. This would sometimes ruin the motor and the therefore the pump. A failure of the second seal prior to failure of the first seal would result in the same problems. By allowing the clean fluid to escape from the cavity, the atmosphere would effectively be on The other side of the first seal, the only remaining working seal. The breaking of seals was a problem since the fluid within had to be maintained at a high pressure to be effective, or at least a pressure higher than the pressure of the liquid being pumped.
Some development in the field created pumps in which the motor was entirely within the pump housing. One type is known as the canned motor pump. Here, the motor could fail for many reasons. Sometimes corrosive liquids would affect the motor. Also, the bearings of the motor as well as other motor parts could clog which increased downtime of the system. This type of pump further was not desirable for use with very hot or dirty liquids. Finally, the efficiency of the system could be lower because the rotating parts of the motor would have to turn within a liquid which caused additional friction during operation. Even higher friction forces occurred because sleeve bearings had to be used instead of ball bearings, since the liquid pumped filled the bearing area.
The use of magnetic pumps was an attempt to solve many of the problems by having the pump housed entirely within a single body and driven by a motor surrounding the body. The motor and pump are magnetically coupled, one magnet is attached to the motor and a magnet of opposite polarity is attached to the pump within the body. However, the magnet pump has the same problem as the canned motor pump with respect to the bearings also exposed to the liquid which is being pumped by the machine. Furthermore, the magnetic pump often generates a lot of heat which is difficult to cool sufficiently to prevent meltdown of the pump. The efficiency in operating a magnetic pump can be quite low because of the loss of energy in transferring the motor movement magnetically through the body to the pump shaft.